There seems to be a 'rule' that you need to hook with the first words/sentence. As with all rules, it's advice. I'm here to disprove that it's a rule, as some books simply don't call for someone to end up dead or whatever in the first sentence. Some are slow builds and character driven.
It is, however, good advice. You do want to grab the reader's interest as quickly as possible or else there's no reason for them to read on, but you shouldn't feel you need to do it with the very first sentence by any means. Sadly though, I don't have a good estimate as to how long you normally have. First page/paragraph - the end of the first chapter? I'm just guessing here. (I'd quite heavily lean towards the first page/paragraph though, as it varies from reader to reader on how much time they'll give you.)
Really though, before I continue, what you need to ask yourself when writing something and wondering whether the hook needs to come in the first sentence, is 'if a reader cannot read beyond the first sentence, are they the audience I need to worry about with this particular book?'
With that out of the way, let me show some excerpts from published works (opening paragraphs) and people have to remember you also have the book blurb/description to fall back on as well:
Pride and Prejudice, by Jane Austen
'It is a truth universally acknowledged, that a single man in posession of a good fortune, must be in want of a wife.'
Rebecca, by Daphne De Maurier (isn't there a 'rule' where you shouldn't start with a dream, by the way?)
'Last night I dreamt I went to Manderley again. It seemed to me I stood by the iron gate leading to the drive, and for a while I could not enter, for the way was barred to me. There was a padlock and a chain upon the gate. I called in my dream to the lodge-keeper, and had no answer, and peering closer through the rusted spokes of the gate I saw that the lodge was uninhabited.'
Pegasus, by Robin McKinley (despite the character having a pegasus, I'm not sure I'd treat that as starting with a bang by any means)
'Because she was a princess she had a pegasus.'
The Lord of the Rings, by J. R. R. Tolkien (granted, you could say that it's interesting that someone's going to be celebrating their 111th birthday, but it's a fantasy story. Not that interesting. You could also say the 'part of special magnificence' is interesting, but really, is it very hook'ish?)
'When Mr. Bilbo Baggins of Bag End announced that he would shortly be celebrating his eleventy-first birthday with a party of special magnificence, there was much talk and excitement in Hobbiton.'
Wizard's First Rule, by Terry Goodkind (first published work and a bit of a dull opening if you ask me with it talking about vines)
'Dusky variegated leaves hunkered against a stem that wound in a stranglehold around the smooth trunk of a balsam fir. Sap drooled down the wounded bark, and dry limbs slumped, making it look as if the tree were trying to voice a moan into the cool, damp morning air. Pods stuck out from the vine here and there along its length, almost seeming to look warily about for witnesses.'
Watchers, by Dean Koontz
'On his thirty-sixth birthday, May 18, Travis Cornell rose at five o’clock in the morning. He dressed in sturdy hiking boots, jeans, and a long-sleeved, blue-plaid cotton shirt. He drove his pickup south from his home in Santa Barbara all the way to rural Santiago Canyon on the eastern edge of Orange County, south of Los Angeles. He took only a package of Oreo cookies, and a large canteen full of orange-flavored Kool-Aid, and a fully loaded Smith & Wesson .38 Chief’s Special.'
Twilight, by Stephenie Meyer (I know this example will be looked upon favorably, but it's valid)
'MY MOTHER DROVE ME TO THE AIRPORT WITH THE WINDOWS rolled down. It was seventy-five degrees in Phoenix, the sky a perfect, cloudless blue. I was wearing my favorite shirt— sleeveless, white eyelet lace; I was wearing it as a farewell gesture. My carry-on item was a parka.'
Fifty Shades of Grey, by E L James (again, another example that will be looked upon favorably.)
'I SCOWL WITH frustration at myself in the mirror. Damn my hair— it just won’t behave, and damn Katherine Kavanagh for being ill and subjecting me to this ordeal. I should be studying for my final exams, which are next week, yet here I am trying to brush my hair into submission. I must not sleep with it wet. I must not sleep with it wet. Reciting this mantra several times, I attempt, once more, to bring it under control with the brush. I roll my eyes in exasperation and gaze at the pale, brown-haired girl with blue eyes too big for her face staring back at me, and give up. My only option is to restrain my wayward hair in a ponytail and hope that I look semi-presentable.'
And just for kicks, while it's from Dean Koontz, do you know that 'rule' about not starting with the weather?
The Taking, by Dean Koontz
'A FEW MINUTES PAST ONE O’CLOCK IN THE morning, a hard rain fell without warning. No thunder preceded the deluge, no wind. The abruptness and the ferocity of the downpour had the urgent quality of a perilous storm in a dream.'
And have you heard that 'rule' about not starting with dialog? (I've got an opening I'm rather fond of myself that starts with dialog, but I'm nowhere near ready to write the story yet.)
The Vision, by Dean Koontz (admittedly, it's not something that would normally be said, but still)
'‘Gloves of blood.’'
Origin, by J. A. Konrath
'“Where is it?” Theodore Roosevelt asked John Stevens as the two men shook hands. Amador, Shonts, and the rest of the welcoming party had already been greeted and dismissed by the President, left to wonder what had become of Roosevelt’s trademark grandiosity.'
Blaggard's Moon, by George Bryan Polivka
'"On a post. In a pond."'
So, what all of this comes down to is it's best to not take anything you read online as rules. The best bet, when you're stuck, is to pick up a book off your bookshelf and read. That's not to say there's absolutely no value in what's said though. Like I said above, it's good advice about hooking from the first sentence, but it depends on what you're writing. Just grab the reader's interest as quickly as possible. Also, do your best to not let people influence your writing, by trying to 'coach' you or 'tell' you how things are done.
Ultimately, just pick the starting point that feels right 'to you'. Not to others.
And to end this, I'll leave you with a link to another blog post on someone else's blog that discusses opening lines. There's also this link.